Device for amplitude modulation of ultra short waves



K. s. KNOL L 2,775,739

DEVICE FOR AMPLITUDE MODULATION OF ULTRA SHORT WAVES Filed March 15, 1952 INVENTORS Kornelis Swier Knol Gerri? De Vries Agenr United States Patent DEVICE FOR AMPLITUDE-MODULATION OF ULTRA SHORT WAVES Kornelis Swier Knol and Gerrit de Vries, Eindhoven, Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application March 15, 1952, Serial No. 276,858

Claims priority, application Netherlands April 6, 1951 4 Claims. (Cl. 33257) The invention relates to devices for the amplitudemodulation of ultra-short waves, wherein the modulation is performed externally of the wave generator.

The invention has for its object to provide a device whereby an effective amplitude modulation is obtained without influencing the generator such that frequency modulation occurs. I

For the modulation of a sharply directed beam of electromagnetic waves it has been suggested to provide a screen of gas discharge tubes in the path of the waves between the transmitter and the receiver, the ionization condition of these tubes being varied with the modulation voltage. In this case only that portion of the radiation which passes through the screen is modulated. Consequently, the energy which is allowed to pass through the screen is in part absorbed by the screen and otherwise reflected in the direction of the transmitter. The latter portion of energy changes theload of the transmitter. It has thereforebeen further suggested to arrange the screen at an angle to the axis of the beam of slightly less than 90, so that the reflected energy can no longer reach the transmitter. This arrangement provides efiective modulation only, if use is made of a screen having a large surface and hence a large number of discharge tubes.

With ultra short-wave transmitters, in which the energy is conducted through a hollow wave guide to the aerial, it is also known to obtain a modulation of the oscillations emitted by the aerial by providing the wave guide with a glow discharge tube, the ionization condition of which is varied by the modulation voltage. In this case the electrodes of the tubes in the Wave guide must be arranged in a manner such that the rod-shaped, conductive parts of the electrodes are at right angles to the electrical field lines in the guide i. e. to the course of these lines in the absence of these conductive parts, so that the course of the field is affected as little as possible. However, it is, as a rule, possible to close a hollow wave guide by means of a wall of energy-absorbing material in a manner such that interfering reflections are avoided. In this case the choice of the specific conductivity of the wall material is extremely critical, at least if the wall is at right angles or substantially at right angles to the axis of the hollow wave guide. Consequently, in these devices a great portion of the energy from the generator will also be reflected, so that the amplitude modulation will be attended by a strong frequency modulation.

It is also known that a substantially reflection-free closure of a hollow wave guide may be obtained, if a keyshaped element of semi-conductive material is used as a closing member, the top of which points towards the side of the guide from where the waves originate. If only the apex is sufficiently small, the choice of the conductivity of the key material is no longer very critical.

The idea underlying this invention is to modulate ultrashort waves in amplitude in a hollow wave guide with the use of one or more gas discharge tubes, arranged in the wave guide, the ionization condition of these tubes being controlled by the modulation voltage. The invention resides in an arrrangement wherein the ionization sphere lies in a plane which is at an angle of not more than 30 to the axis of the wave guide. The embodiment is preferably such that the axes of the gas discharge tubes themselves are at an angle of not more than 30 to the axis of the wave guide. It will, in general, be possible to obtain an effective modulation with the use of a single gas discharge tube having a comparatively small sectional area. In a preferred embodiment the wave guide has a rectangular sectional area and the axis of the gas 7 discharge tube lies in a plane which is parallel to the shorter side of the wave guide.

With the device according to the invention the modulation is mainly due to the fact that in accordance with the ionization condition of the gas a greater or smaller portion of the energy conducted through the wave guide is absorbed by the ionized gas atmosphere. Owing to the oblique position of the discharge tubes, reflection of energy will substantially not occur, so that the generator is substantially not affected by the modulation.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing.

Fig. 1 of the drawing is a greatly simplified perspective view of the device according to the invention;

Fig. 2 is a cross-sectional view of part of the device, comprising a circuit to vary theionization condition of the gas atmosphere in the tube or tubes; and

Fig 3 shows in section a modified arrangement of the gas discharge tubes.

Referring now to Fig. 1, reference numeral 1. designates a generator for ultra-short waves, which is connected to a hollow wave-guide 2 of rectangular cross-sectional area, in which occur, for example, waves of the Ho-type. The electrical field lines are at right angles to the upper and the lower surfaces; the right-hand end of this hollow wave-guide may be connected to a load such as an aerial. This end may, for example, merge in a hornshaped aerial.

In order to obtain modulation of the energy transmitted through the wave-guide to the aerial, provision is made of a gas discharge tube 3 of elongated shape, comprising electrodes 4 and 5, the tube being introduced into the wave-guide through apertures in the upper and lower walls, the axis of the tube being at an angle of not more than 30 to the axis of the wave-guide. The ionized gas atmosphere has a damping efiect on the field, of which the electrical lines of force are at right angles to the upper wall and the lower wall of the hollow wave-guide.

By varying the ionization condition of the gas atmosphere in the tube 3 in accordance with the modulation voltage, the energy emerging at the right-hand end is amplitude-modulated in accordance with this modulation voltage. Satisfactory results are obtainable with the use of a wave-guide of about 12 mms. in height and 24 mms. in width, comprising a gas discharge tube of about 6 mms. in internal diameter. If desired, a number of discharge tubes may be arranged side by side, their axes lying in a plane at an angle of not more than 30 to the upper and the lower surfaces of the wave-guide and preferably at right angles to the side surfaces. In general, a single tube having suitable properties will suflice. The advantage of the use of a single tube, the axis of which lies in the central plane of the wave-guide is that the field course is only slightly disturbed by the presence of the apertures in the upper and the lower walls of the wave-guide, since in the center of the guide no cuirent component prevails at right angles to the axis of the guide.

Fig. 2 shows part of the device in a sectional view. The discharge tube '3 is "fed through aseries resistor 7' from a current source 6. The ionization condition of the gas atmosphere is modulated by a low-frequency signal by connecting the series combination of an inductor 8 and a capacitor 9 in parallel with the series resistor 7. The low frequency signal is transmitted inductively through an inductor 10 to the inductor 8. This modulation method is known per se.

As is shown in Fig. 3, a number of gas discharge'tubes may, as an alternative, be arranged with their axes at right angles to two opposite walls of a wave-guide of rectangular sectional area, the arrangement being such that the plane containing the axes of the tubes is at an angle of not more than 30 to'the two other side surfaces of the wave-guide.

The gas discharge'tubes may be completely housed inside the wave-guide, in which case only the current supply wires need be taken through the wall. In this case it will often be necessary to take measures to avoid disturbing the field course by conductive parts in the waveguide.

The smaller the angle at which the plane of the ionized gas atmosphere is inclined relative to the axis of the waveguide, the smaller willbe the reflection. From'a practical standpoint, a very useful value for this angle is 10". It is found that an advantageous modulation characteristic is obtained, if the conditions are chosen to be such that, on an average, half of the energy is absorbed.

What we claim is:

1. Apparatus for amplitude modulating ultra-short wave energy comprising a hollow wave-guide for conveying said energy, at least one elongated gaseous discharge tube disposed in said guide in the path of said energy, and means coupled to said tube to vary 'the ionization thereof in accordance with a modulating'signal, said tube occupying substantially less than the total cross-section area of said waveguide and being disposed with respect to said guide at an angle at which the ionized gas atmosphere therein is primarily located in a plane lying at an 4 angle not exceeding 30 with respect to the axis of the guide.

veying the energy produced therein to a load, at least one gaseous discharge tube of elongated form extending through the walls of said guide at an angle at which the gas atmosphere therein lies in a plane with respect to the axis of theguide which does not exceed 30, said tube being sufficiently narrow so as to occupy substantially less than the total cross-section area of said wave-guide, and means coupled to said tube to vary the ionization thereof in accordance with a modulation signal.

3. Apparatus for amplitude modulating ultra-short wave energy comprising a hollow wave-guide of rectangular cross-section for conveying said energy, a single elongated gaseous discharge tube disposed at a position in which the axis of the tube is located in the central plane of the wave-guide and parallel to the side walls having the smaller transverse dimensions, said tube being sufficiently narrow so asto occupy substantially less than the total cross-section area of said- Waveguide, and means coupled to said tube to vary the ionization thereof in accordance with a modulating signal, said tube being disposed at a position at which the ionized gas atmosphere therein is primarily located in a plane lying at an angle not exceeding 30 with respect to the axis of the guide.

4. Apparatus, as set forth in claim 3, wherein said angle is substantially 10 to the axis of the guide.

References Cited in the file of this patent UNITED STATES PATENTS 2,047,929 Linder July 14, 1936 2,106,770 Southworth et al Feb. 1, 1938 2,173,234 Linder Sept. 19, 1939 2,557,180 Fiske' June 19, 1951 2,716,192 Johnson Aug. 23, 1955 OTHER REFERENCES Technique of Microwave Measurements by Montgomery, Radiation Laboratory Series No. 11', pages 199, 200. 

